This invention relates to a system and method for detecting and locating defects such as delaminations and cuts in various types of tires.
Over twenty five million retreaded tires were sold in the United States in 1995 alone. Retreaded tires are used in critical applications such as for military and commercial aircraft and even school buses. As such, when a retreaded tire fails due to a delamination, the result can be disastrous.
There have been numerous attempts made to devise inspection devices which can detect defects present in a tire after the tire has been retreaded. Such prior art devices, however, suffer from numerous drawbacks. Holography methods, which map tire defects using optical means are extremely expensive, generally about $500,000 per machine, are extremely sensitive to vibration, and also relatively slow. Shearography is a similar technique less sensitive to vibration, but also very expensive and slow. High voltage tests are less expensive, but are limited to detecting only metal inclusions in the tire carcass. Ultrasound techniques are capable only of detecting air-filled defects, are very sensitive to the thickness of the tire wall being measured, and therefore are not generally used to inspect aircraft tires. The air injection test specified by military specification-MIL-R-7726H, supplement 1, is extremely time consuming and not very accurate.
Since aircraft tires are retreaded on average five to seven times and in some cases up to twelve times, any inspection device which is overly expensive and complicated and/or which involves long inspection times, and/or which does not accurately detect different types of defects, will not be commercially viable. There are over 1,400 retreading plants in the United States alone. Therefore, the sheer cost of most prior art inspection equipment makes it impractical to inspect all retreaded tires.
It is therefore an object of this invention to provide a tire defect detection system and method which accurately detects defects in a tire at a low cost and with short inspection times.
It is a further object of this invention to provide such a tire defect detection system and method which is capable of inspecting 100% of the tire thereby increasing the reliability of the test.
It is a further object of this invention to provide such a tire defect detection system and method which eliminates operator subjectivity.
It is a further object of this invention to provide such a tire defect detection system and method which incorporates many commercial off-the-shelf components.
It is a further object of this invention to provide such a tire defect detection system which is not susceptible to the effects of tire side wall lettering or other molded features.
It is a further object of this invention to provide such a tire defect detection system and method which increases the margin of safety which can be expected from a retreated tire.
This invention results from the realization that tires can be inexpensively and comprehensibly inspected and tested both before and after retreading and even in situ by impacting the tire with an actuator and analyzing the resulting sound wave for evidence of delamination and other faults. A neural network or other non-linear correlator is trained to recognized delaminations and other tire defects based on a number of discriminator quantities computed for each resulting sound wave and to average out non-defect information such as surface lettering, wall thickness, and the like.
This invention features a tire defect detection system comprising a support structure for receiving a tire; an actuator disposed proximate the tire for impacting the tire; a microphone disposed proximate the actuator for receiving a sound wave generated when the actuator impacts the tire; and a computer responsive to the microphone. The computer includes means for calculating a plurality of discriminator quantities from the sound wave, and means for comparing the calculated discriminator quantities with stored discriminator quantities indicative of a defect to determine whether a defect is present in the tire.
The actuator typically includes a solenoid. Further included is a controller for driving the solenoid at an impact time of less than 0.1 seconds. The microphone has a response of approximately 70-15,000 Hz.
The computer preferably includes a routine for performing a FFT transform for each waveform. From this sound wave and the FFT transform, a plurality of discriminator quantities are calculated: the mean value of the amplitude of the FFT transform in each frequency band; the standard deviation of the amplitude of the FFT transform in each frequency band; as the root mean square of the sound wave; the inverse of the autocorrelation time of the sound wave; the average area under each excursion above the standard deviation of the sound wave; the average time between crossings of one standard deviation above the mean; the average time between crossings one standard deviation above the mean and one standard deviation below the mean; the average length of the sound waveform; the average length of the sound waveform above one standard deviation above the mean; and the width of any pulse of the sound waveform which are above one standard deviation above the mean as a discriminator quantity.
The defect detection system further includes means for constructing a defect transfer function based on the calculated discriminator quantities.
The tire defect detection system includes an actuator for impacting the tire; a microphone disposed proximate the actuator for receiving a sound wave generated when the actuator impacts the tire; and a computer responsive to the microphone. The computer is programmed to perform a FFT transform for the sound wave, calculate a plurality of discriminator quantities from the sound wave and the FFT transform, and construct a defect transfer function based on the calculated discriminator quantities to provide an indication of whether a defect is present in the tire.
The method of inspecting tires for defects, in accordance with this invention, includes obtaining a tire with known defects; impacting the tire with an actuator; recording the resulting sound waves; calculating a plurality of discriminator quantities for each sound wave, constructing a defect transfer function based on the calculated discriminator quantities and storing the coefficients thereof. A tire with unknown defects is then impacted with an actuator; the resulting sound waves are recorded; a plurality of discriminator quantities for each sound wave are calculated; and the stored coefficients of the constructed defect transfer function are used to determine whether a defect is present in the tire with unknown defects.